What would be the most efficient way to make a spiral curve for objects shaped as in the photo?
Extend the spiral's lenght extruding the curve's points. Add a Bezier Circle in the same position as the spiral is. Scale it to give a curve a desired thickness. Apply its transformations (Ctrl+A-->Rot&Scale).
To make make it gradually thinner in the inside select the very top point of the curve and scale it a bit with Alt+S using the proportional editing. Then scale it with S with a big proportional size falloff.